The present invention relates to a rechargeable battery pack of the type commonly used in video recorders, cellular telephones, power tools and the like. Even more particularly, the present invention relates to an overcharged protection circuit for a battery pack. The invention also relates to methods of recharging such battery packs.
A battery capable of being recharged after a battery is discharged through use, is commonly called a rechargeable battery. Presently, rechargeable batteries are becoming ubiquitous in present day society, and it is advantageous that the rechargeable battery always be charged to maximum capacity. Conventionally, a rechargeable battery is charged by using a base unit plugged into a wall outlet, or for smaller rechargeable batteries, by using a vehicle mounted adapter. A charger usually operates in either a standard mode, having a charging rate in coulombs (amps/second) dependant on the characteristics of the rechargeable battery, or in a trickle mode at a charging rate substantially less than in the standard mode. Until recently, most rechargeable batteries have included one or more rechargeable nickel cadmium (NI-CAD) cells comprising a rechargeable battery pack. Each of these battery packs have two external contacts for contacting the positive and negative terminals of a charging adapter.
When the user inserts a rechargeable battery into a conventional charging adapter system, the insertion of the battery is recognized and charging automatically begins. Unfortunately, when the user inserts a fully charged battery into the charging adapter, the battery voltage can actually decrease. (A decrease in battery voltage will be referred to herein as generating a -.DELTA.V, and a voltage increase will be referred to herein as generating a .DELTA.V.) In addition, overcharging of a rechargeable battery can, also result in a decrease in battery voltage -.DELTA.V. Furthermore, when a vehicle mounted adapter is used, a fully charged battery may be charged too frequently, for example each time the vehicle is started, also resulting in a decrease in battery voltage. The decrease in voltage has a memory effect in the rechargeable battery which is increased due to frequent charging, thereby reducing the lifetime of the battery.
As shown in FIG. 1 and 2, when a NI-CAD battery pack is charged by a charging adapter, the voltage across the two contacts of the battery pack generally rises until it reaches a peak voltage and then the voltage of the battery pack begins to decrease. Meanwhile, when the NI-CAD battery pack is being charged, the temperature of the battery pack initially increases relatively slowly but increases dramatically over time. There have been several attempts to produce charging adapters which terminate the high rate of charging at the peak charge, prior to the substantial voltage decrease stemming from overcharging. Early attempts were directed to charging adapter systems which terminated the high rate of charging based upon the temperature of the rechargeable battery pack. These systems included a thermostatic switch located in close proximity to the battery pack which measured the temperature of the battery. Once the battery pack heated to a predetermined temperature, the thermostatic switch would sense the battery temperature and terminate the high rate of charge to the battery pack.
More recent attempts to terminate the charging of a battery pack near the peak charge are based on the detection of a decrease in voltage (-.DELTA.V) in the battery pack. The high rate of charge is terminated when the voltage peaks at a maximum and then drops by a predetermined voltage level, as shown in FIG. 1 as -.DELTA.V.
Such conventional charging adapters work sufficiently well with NI-CAD battery packs. However, these charging adapters, which terminate the high rate of charging based upon a -.DELTA.V, do not prevent overcharging of more recently developed forms of battery packs such as nickel metal hydride rechargeable battery packs. As shown in FIG. 3, the voltage across the two contacts of a nickel metal hydride battery pack increases gradually. Immediately prior to peak voltage, the rate of voltage increasing over time itself increases. This change in the rate of voltage increase will hereinafter be referred to as .DELTA..sup.2 V/.DELTA.t.sup.2. As understood by those skilled in the art, .DELTA..sup.2 V/.DELTA.t.sup.2 represents the second derivative of the measured voltage across the contacts of the battery pack with respect to time. After the nickel metal hydride battery reaches peak charge, the battery pack voltage drops relatively slowly. As seen by a comparison of the plots of FIGS. 1 and 3, nickel metal hydride battery packs do not tend to drop in voltage nearly as quickly as a NI-CAD battery pack because typical nickel metal hydride battery packs take longer to drop in voltage after obtaining peak charge. Thus, conventional adapters do not terminate the high rate of charge quickly enough to prevent overcharging of the nickel metal hydride battery. Furthermore, as shown in FIG. 3, the temperature of a nickel metal hydride battery continues to rise steadily during charging. If a charging adapter does not terminate its charge soon after a nickel metal hydride battery reaches peak charge, the battery can be damaged from overheating. Accordingly, it has become generally preferred to terminate the high rate of charge to a nickel metal hydride battery pack by sensing the battery pack temperature and terminating the high rate of charge when the temperature reaches a threshold temperature. This necessitates the need for separate charging adapters for NI-CAD battery packs and nickel metal hydride battery packs resulting in a duplicity of costs. Thus, it would be advantageous if both NI-CAD battery packs and nickel metal hydride battery packs could be charged from a single charging adapter.
To this end, U.S. Pat. No. 5,708,350 discloses a nickel metal hydride pack which may be charged from the conventional charging adapters originally intended for charging NI-CAD battery packs. The nickel metal hydride battery pack includes an overcharge protection circuit which artificially transmits an increased .DELTA.V to the charging adapter once the nickel metal hydride pack reaches a threshold temperature. The overcharged protection circuit includes a temperature responsive switch coupled between the battery pack contacts and the rechargeable cells. Upon the temperature response switch reaching a threshold temperature, the switch adjusts the voltage appearing across the contacts of the rechargeable battery by an amount exceeding the predetermined change in voltage to which the adapter responds. The voltage across the contacts is increased by inserting a resistor into the current path. The resistor acts as a current limiter which eventually drops the battery temperature below the temperature which caused the switch to activate. Once the battery temperature drops below this predetermined threshold level, the thermostatic switch removes the resistor from the circuit and the voltage appearing across the contacts of the battery pack artificially drops. Thus, the overcharged protection circuit of U.S. Pat. No. 5,708,350 tricks a conventional charging adapter to terminate the high rate of charge even though the requisite voltage drop appearing across the battery contacts may not have occurred.
This system suffers from several drawbacks. First, where the battery pack has been placed in a cold environment prior to insertion into the charging adapter, the thermostatic switch will not operate properly when the nickel metal hydride battery reaches peak voltage. Furthermore, depending on the ambient conditions surrounding the charging adapter, and the characteristics of the thermostatic switch, it may take an unacceptably long time for the thermostatic switch to both artificially increase the battery pack voltage and then artificially decrease the battery pack voltage resulting in overheating and overcharging of the nickel metal hydride battery pack.
Thus, there exists a need for a system enabling NI-CAD and nickel metal hydride battery packs to be charged in the same charging adapter. It would also be highly desirable to provide a nickel metal hydride type battery pack which may be effectively charged in conventional adapters which specialize in charging NI-CAD battery packs.
It would also be highly desirable to provide a nickel metal hydride-type battery pack including an overcharge protection circuit which has its high charge rate terminated upon reaching its peak charge.